Control units which control the operation of internal combustion engines frequently are sealed to prevent ingress of moisture, humidity, salt spray and the like, which might interfere with proper operation of the control units. In operation, the control units receive input signals from sensors located at various operating elements of the internal combustion engine (ICE), for example signals representative of speed, loading on the engine, ambient temperature, engine temperature and the like. To test such units, it has been customary to apply to the control units signals which are the same as those derived from the sensors. For example, speed sensors, acceleration pedal sensors, temperature sensors and the like, are connected to the inputs of the control unit; the outputs of the control units are then coupled to the respective positioning elements which, in turn, actually position the respective operating elements of the ICE. Thus, the unit is tested to operate as if the vehicle were actually operating and moving. Test stands of this type are expensive, space-consuming, and have the disadvantage that testing of the electronic control unit is time-consuming due to the connection time required for the respective input and output elements and the calibration time therefor. Additionally, it has been found that the respective test methods and apparatus provide, similar to actual operation of the engine, simultaneously effective data from the inputs as well as the outputs. The control unit may jointly process at least some of those data. It may thus occur that a malfunction which is due to a connection or internal element of the control unit and which is peculiar to a single input or a single output is masked by influences derived from other inputs or applied to other outputs, or at least highly attenuated. Malfunction at specific input or output ports of the control unit thus is difficult to diagnose, since desired specific characteristics are difficult to obtain. Additionally, it is frequently necessary to invade the internal components or connections of the control unit. The control unit, however, preferably, should remain sealed and in its original condition. Opening the control unit for making connections, and then resealing the control unit, may cause additional defects due, for example, to improper or ineffective resealing, or unintended destruction of components or connections.